Magnesium is the eight must abundant element in the earth's crust. This lightweight metal is used in many applications and recent change in emission norm by automotive industry has created a regain in the demand.
Over 75% of the primary production of magnesium is actually made by thermal process and present major environmental concern and high-energy consumption. Electrolytic route of production is also use and is generally made by electrolysis of salt from sea water or dead sea. Salt from Dead Sea generally contain less that 3.5% of magnesium.
One of the difficulties related to the use of salts is to isolate the magnesium chloride from the rest of the feed in other to produce a pure magnesium salt to be introduced into an electrolytic cell for example.
The exploitation of important deposits of serpentine for the asbestos fiber in the last decades generated huge quantities of tailings. This ore consist of more than 90% serpentine (also known as magnesium iron silicate hydroxide), mainly as lizardite Mg3Si2O5(OH)4 with minor antigorite (Mg, Fe)3Si2O5(OH)4, brucite Mg(OH)2, magnetite Fe3O4, awarite Ni8Fe3, traces of chromite Fe(Cr, Fe)2O4 and chromium-rich spinel (Cr, Fe, Al, Mg)3O4.
The asbestos tailings contain between 23-27% of magnesium and can be extracted to produce pure magnesium. They also contain around 38% SiO2, 1-6% Fe, 0.2-0.3% Al and 0.1-0.2% Ni. Trace amounts of others elements are also present.
Several hydrometallurgical and electrolytic processes were developed for magnesium bearing ore but none of those processes is in production at this time due to difficult operation conditions. In general the resulting magnesium chloride still contains significant amounts of impurities that must been removed before being consider as a feed material for the magnesium electrolysis production. Those impurities can conduct to a poor cell performance and result in low current efficiency. Also, those processes have for only objective to produce magnesium chloride or metallic magnesium to the detriment of secondary products with commercial value.
In the past, a method has been proposed to produce a magnesium chloride solution from siliceous magnesium minerals (U.S. Pat. No. 5,091,161). The method involves leaching the material with a hydrochloric acid solution at a temperature above 50° C. The pH is maintained below 1.5 to prevent the formation of silica gel. The leaching can be carried out in a continuous manner. The pH of the leaching solution is increased to 4.0-4.5 with magnesia to precipitate the bulk of impurities followed by solid/liquid separation to obtain magnesium chloride liquor cleansed. A second step of purification at pH 6.0-7.0 with caustic soda and chlorine gas allows to oxidize and to precipitate the residual iron and manganese. A last stage of purification is made by ion exchange column to remove trace amounts of impurities such as nickel and boron.
In a same manner, WO 2000/017408 proposes a method to produce a magnesium chloride solution from magnesium containing materials, but with a single step of impurities separation.
These processes represent a significant step forward over those known previously, but still have some disadvantages. For example, the use of caustic magnesia for iron impurity removal is costly and imposes a heavy economic burden. Also, these processes do not allow recovering the silica for future sale because it is contaminated by iron and other impurities, including nickel. Although these processes contain stages of purification, they do not allow eliminating some impurities, such as sulfates. It is known that sulfate introduction in the electrolytic cell incurs a drop of current efficiency by increasing the voltage.
To remove iron impurity from magnesium chloride brine, hydrolysis methods have been propose such as described in the WO 2014/029031 and WO 2009/153321. Briefly, the brine is first concentrated and oxidized where ferrous chloride is converted to a ferric and oxide form. Ferric chloride is subsequently hydrolyzed generating hematite and hydrogen chloride. The following reactions describe the oxidation and hydrolysis steps.6FeCl2+3/2O2→4FeCl3+Fe2O3 FeCl3+3/2H2O→3HCl+½Fe2O3 
While recovery of hydrochloric acid and hematite may be achieved using these processes, its application tends to be limited to liquors containing only ferrous/ferric chloride. When other chloride are present in large quantity in solution, for instance magnesium chloride, the activity of the chloride ions and proton tend to be too high to permit the proper functioning. Such process will work in the laboratory in batch mode but not in a continuous mode because the magnesium chloride concentration increase relative to that of iron, then the solution freezes and becomes solid. Moreover, hydrolysis method is conducted under pressure and at elevated temperature, around 200° C. It requires expensive equipment and also consumes a lot of energy. Also, the hydrolysis of a brine containing some magnesium conduct to poor purification efficiency for iron. It was observed a loss of about 6 to 11% of the magnesium while removing only 62 to 70% of the iron. Thus, hydrolysis is not a selective method and further purification steps are required. Therefore, this method cannot be viable economically for large volumes.
To concentrate a magnesium chloride brine to obtain a hydrate salt, evaporation is currently used. However, this method requires a lot of energy and consequently an important cost. For this reason, the use of evaporation must be limited.
In a same way, the electric consumption of the factories of magnesium by electrolytic process comes mainly of the electrolysis step by the decomposition voltage of the MgCl2. This consumption thus has a significant impact on production cost and the profitability.
By conventional electrolysis process of magnesium in molten salt, the carbon anodes tends to decompose by reaction with Cl2 emitted to form organochlorine compounds, which have a negative environmental impact. The life time of the anode is also reduced.
Accordingly, there is thus still a need to be provided with an improved global process for producing magnesium metal from magnesium-bearing ores such as asbestos tailings and to improve the overall economic by generating valuable by-product, limiting the purchase of chemical base, reducing the energy consumption and restricting the organochlorine emissions.